Doppler radar systems are the principal measuring instruments used to control and enforce legal speed limits for vehicles.
For such instruments to be approved for the intended use, it must be ensured and demonstrated that the speed measured is correct, on the one hand, and it must be possible to link the speed measured beyond doubt to the detected vehicle, on the other hand.
The present invention relates to a method which solves, in particular, the problem of establishing such a link beyond doubt but which, in this context, also makes the speed measured plausible.
The fact that there are measuring situations in which it is not possible to link, beyond doubt, the speed measured to the detected vehicle is, in particular, attributable to the principle of radar technology. The radar beam generally forms a radar cone which spreads at an angle of approximately 5-10° and increases in width as the distance increases, which means that when the instrument is used on a multiple lane road, several vehicles may be within the radar cone (measuring range) at the same time.
To identify a detected vehicle, i.e., a vehicle that reflects the radar beam, beyond doubt, i.e., to recognize the detected vehicle in a group of vehicles, EP 0 93 5 764 B 1 proposes to detect, simultaneously with the speed, the distance as well and to link the vehicle by way of the detected distance to a lane.
To measure the distance by means of radar technology, it is known to use pulse radar systems and radar systems with frequency-modulated continuous wave radar signals. In both cases, the distance is not actually measured but derived from other measured variables. Thus, in the following description, any reference to measured values and to a measured speed is meant to be understood as a derived speed. This also applies to the distance and the angle.
Pulse radar systems determine the distance by way of measuring the travel time and require an extremely accurate time measurement in order to be able to measure the distance, i.e., the radial distance from the reflecting vehicle parts to the radar antenna. The resolution of a distance of, e.g., one meter requires a time resolution of 3.33 nsec. For high resolutions, a sequence control of the measuring process therefore entails a high degree of technical complexity.
Continuous wave radar systems determine the distance by way of the phase difference of reflected radar signals of different frequencies.
It is known that a continuous wave radar system transmits a continuous wave radar beam of constant amplitude and frequency. When reflected by a moving object, i.e., a vehicle, this radar beam undergoes a frequency shift as a function of the speed of the vehicle. The beam portion that is reflected back into the radar system or to the radar antenna is compared to the emitted radar beam, and a frequency difference, the so-called Doppler frequency, is calculated, which is proportional to the speed of the vehicle.
With the emission of a radar beam in different frequencies, frequency-shifted reflection beams are obtained, from the phase difference of which the distance is derived. The speed and the distance are thus determined in a single measuring step, which links the measured values unequivocally to each other.
In contrast to the measurement of the speed which can be measured extremely accurately with the Doppler radar principle, the range of variation of the measured distances is very large. The point reflections that come from a vehicle and arrive at the radar antenna extend to the entire vehicle contour on which the radar cross section is projected. The radar cross section that is projected onto a vehicle which moves through the radar cone changes as a function of the specific geometry of the vehicle as well as of its position in the radar cone, beginning from the moment the vehicle enters until it exits the cone. At each measuring moment, the receiver detects a sum of measured values (group of measured values) from partial reflections. Statistically, this sum, together with other parasitic reflectors, such as guard rails or metal fences, is as a rule treated as a Rayleigh distribution. Distances which scatter on the order of the width of the lane and the vehicle dimensions are measured.
An unequivocal identification of a vehicle in a group of vehicles solely on the basis of individual distances measured which are linked to a lane is not possible because of the potential ambiguities due to different reflection properties of the vehicles and potential multiple reflections.
Even if at each measuring moment a mean value is generated from the sum of the measured values, this mean value cannot always be unequivocally linked to a lane.
This is also the case if measured values outside a specified range of tolerance are ignored and the mean value is generated solely from the remaining values measured. Although this measure excludes a the risk of falsifying the average distance measured due to partial reflections from parasitic reflectors, an identification based on the mean distance measured and its link to a road is not possible for each measuring situation.
The radar system would necessarily have to be oriented at such an acute angle relative to the road so that all conceivable distances measured can be unequivocally linked only to one lane. Since due to the width of the radar cone, the vehicles necessarily move through a specific distance range, these distance ranges for two vehicles that are to be distinguished from each other, must not overlap each other. However, the distance ranges shift if the vehicles do no drive exactly in the center of the lane and will overlap every time a vehicle within the measuring range changes lanes.
This leads to measuring situations in which a vehicle cannot be unequivocally linked to a lane, which means that the lane is not a reliable characteristic for identifying a vehicle in a group of vehicles.